Various vehicle behavior control devices are known that control behavior of a vehicle by performing anti-side slip control, such as anti-spin (anti-oversteer) control and understeer suppression control, in the vehicle.
For example, Japanese Patent Application Publication No. JP-A-H9-104329 discloses a technology in which a spin amount is obtained. The spin amount is obtained as the sum of a value obtained by multiplying a side slip acceleration Vyd, which is a side slip index of a vehicle body, by a determined coefficient and a value obtained by multiplying a slip angle β of the vehicle body by a determined coefficient. Alternatively, the spin amount is obtained as a linear sum of the slip angle β of the vehicle body and a slip angle velocity β′(=dβ/dt), which is a derivative value of the slip angle β. When the obtained spin amount exceeds a threshold value, anti-spin control is performed.
Japanese Patent Application Publication No. JP-A-H7-215190 discloses a technology in which the slip angle velocity β′ of the vehicle body and the slip angle β of the vehicle body are expressed by a coordinate in a β-β′ two dimensional state diagram. When the coordinate of the obtained slip angle velocity β′ and slip angle β exceeds the boundary between a stable region and an unstable region of the β-β′ two dimensional state diagram and enters the unstable region, anti-spin control is started.
As described above, the method described in JP-A-H9-104329 compares the spin amount with the threshold value, and the method described in JP-A-H7-215190 compares the coordinate of the slip angle velocity β′ and the slip angle β with the boundary in the β-β′ two dimensional state diagram. If the spin amount or the coordinate exceeds the boundary determined in advance, the anti-spin control is performed. However, there are some cases when spin behavior does not actually occur, for example, when the posture of the vehicle is corrected after changing lane or after performing a steering operation to avoid an obstacle in the case of an emergency. This phenomenon will be described with reference to FIG. 18.
In the case of changing lane or the like, after the driver turns a steering wheel in a direction he/she wants to go, the driver turns back the steering wheel to direct the vehicle in the traveling direction. Accordingly, as shown in FIG. 18, a steering angle, a yaw rate, and a lateral acceleration (lateral G) are generated in accordance with the turning operation and the turning back operation of the steering wheel. In addition, when correcting the posture of the vehicle after turning back the steering wheel, a yaw rate and a lateral G are generated in a direction that is opposite to the direction when the steering wheel is turned back. As a result, the spin amount described in JP-A-H9-104329 or the coordinate described in JP-A-H7-215190 exceeds the threshold value, and thus the anti-spin control is performed.
The above-described coordinate is likely to enter the unstable region particularly when the posture is corrected in a region where the absolute value of the slip angle β is small but the absolute value of the slip angle velocity β′ is large, i.e., a region in the vicinity of the boundary near the β′ axis in the β-β′ two dimensional diagram. In such a case, the anti-spin control is performed.
If the anti-spin control is performed even in such a case, it may give the driver a feeling of erroneous activation or an unpleasant sensation. Further, it may cause an uncomfortable sensation such as kickback to the steering wheel.
In the method described in JP-A-H7-215190, various data about a trajectory showing the changing states of the slip angle velocity β′ and the slip angle β are collected, and the stable region and the unstable region are classified based on the collected data. However, it is difficult to determine whether or not the spin behavior during the vehicle posture correction is occurring.